Re: [PATCH v3 0/6] Provide a fraemework for RISC-V ISA extensions

[Jisheng Zhang <jszhang@kernel.org>]

On Tue, Feb 15, 2022 at 11:06:24AM -0800, Atish Kumar Patra wrote:
> On Tue, Feb 15, 2022 at 8:04 AM Jisheng Zhang <jszhang@kernel.org> wrote:
> >
> > On Tue, Feb 15, 2022 at 01:02:05AM -0800, Atish Patra wrote:
> > > This series implements a generic framework to parse multi-letter ISA
> > > extensions. This series is based on Tsukasa's v3 isa extension improvement
> > > series[1]. I have fixed few bugs and improved comments from that series
> > > (PATCH1-3). I have not used PATCH 4 from that series as we are not using
> > > ISA extension versioning as of now. We can add that later if required.
> > >
> > > PATCH 4 allows the probing of multi-letter extensions via a macro.
> > > It continues to use the common isa extensions between all the harts.
> > > Thus hetergenous hart systems will only see the common ISA extensions.
> > >
> > > PATCH 6 improves the /proc/cpuinfo interface for the available ISA extensions
> > > via /proc/cpuinfo.
> > >
> > > Here is the example output of /proc/cpuinfo:
> > > (with debug patches in Qemu and Linux kernel)
> > >
> > > / # cat /proc/cpuinfo
> > > processor     : 0
> > > hart          : 0
> > > isa           : rv64imafdcsu
> > > isa-ext               : sstc,sscofpmf
> > > mmu           : sv48
> > >
> > > processor     : 1
> > > hart          : 1
> > > isa           : rv64imafdcsu
> > > isa-ext               : sstc,sscofpmf
> > > mmu           : sv48
> > >
> > > processor     : 2
> > > hart          : 2
> > > isa           : rv64imafdcsu
> > > isa-ext               : sstc,sscofpmf
> > > mmu           : sv48
> > >
> > > processor     : 3
> > > hart          : 3
> > > isa           : rv64imafdcsu
> > > isa-ext               : sstc,sscofpmf
> > > mmu           : sv48
> > >
> > > Anybody adding support for any new multi-letter extensions should add an
> > > entry to the riscv_isa_ext_id and the isa extension array.
> > > E.g. The patch[2] adds the support for various ISA extensions.
> >
> > Hi Atish,
> >
> > Thanks for this series. I'm thinking cpu features VS ISA extenstions.
> > I'm converting the sv48 to static key:
> > https://lore.kernel.org/linux-riscv/20220125165036.987-1-jszhang@kernel.org/
> >
> > Previously, I thought the SV48 as a cpu feature, and there will be
> > more and more cpu features, so I implemented an unified static key
> > mechanism for CPU features. But after reading this series, I think
> > I may need to rebase(even reimplement) the above patch to your series.
> > But I'm a bit confused by CPU features VS ISA extenstions now:
> >
> > 1. Is cpu feature  == ISA extension?
> >
> > 2. Is SV48 considered as ISA extension?
> > If yes, now SV48 or not is determined during runtime, but current ISA
> > extensions seem parsed from DT. So how to support those ISA extensions
> > which can be determined during runtime?
> >
> > Could you please share your thought?
> >
> 
> Here are my two cents:
> 
> I think the cpu feature is a superset of the ISA extension.
> cpu feature != ISA extension.
> 
> While all ISA extensions are cpu features, all CPU features may not be
> an ISA extension.
> e.g. sv48 is not a ISA extension but F/D are (used to set the
> cpu_hwcap_fpu static key)
> 
> Moreover, not all cpu feature/ISA extension requires a static key.
> e.g SSTC extension will require a static key because the check has to
> happen in the hot path.
> However, sscofpmf extension don't need a static key as the check
> happens only one time during boot.
> 
> We should keep these two separate but a common static framework would
> be very useful.
> 
> Here is the flow that I have in my mind.
> 1. All ISA extensions will be parsed through riscv,isa DT property
> 2. Any supported/enabled extension will be set in riscv_isa bitmap
> 3. Any extension requiring a static key will invoke the cpus_set_cap.
> 
> cpus_set_cap will be invoked from a different code path that uses a
> static key for a specific ISA
> extension or a CPU feature.
> 
> The only problem I see here is that we have to set a bit in both
> cpu_hwcaps & riscv_isa bitmap.
> We also have to define the value of that bit for any extension
> requiring a static key twice as well.
> 
> I think that should be okay. But I would like to hear what everybody
> else thinks as well.
> 

Thank Atish's input. I notice that SV57 support is merged, I'll
send a new version to apply static mechanism to both SV48 and SV57
once rc1 is released.